In the electrophotographic methods, a method generally employed is to firstly form an electrostatic latent image using various means by using a photoconductive material, such as selenium, as a light sensitive medium, and a toner is adhered to this electrostatic latent image using a magnetic brush development method to develop the latent image.
In the magnetic brush development methods, carriers used for imparting a suitable amount of positive or negative charge to the toner are generally classified into coated carriers and noncoated carriers. The coated carriers are superior when considering the life of a developer, and therefore various types of coated carriers have been developed and used in practice.
Various properties are required in coated carriers, and the particularly important include suitable charging property, impact resistance, wear resistance, good adherence between the core material and the coated layer, and uniformity of charge distribution.
In view of the above requirements, conventional coated carriers have always had problems and required improvements, and no perfected products are known at present. For example, it has been proposed to use fluorinated vinyl polymers as a coated layer material either singly or blended with acrylic monomers. However, the fluorine polymers are deficient in the capacity to charge a toner negatively because of the strongly negative electric negativity of fluorine itself, although it is superior in surface contamination resistance. These characteristics of the fluorine polymers are particularly conspicuous under high temperature and high humidity. Consequently, carriers with an amount of fluorine polymer sufficient for surface contamination resistance either singly or blended at high concentrations with acrylic polymers were low in charging properties, and there were difficulties in reconciling contamination versus the amount of charge under high temperature and high humidity.
Also, carriers coated with the fluorine polymers have slow build-up speeds in their charges under low temperature and low humidity, and have the further defect of poor charge exchangeability between toners and carriers. This defect is conspicuous when they are used in combination with a color toner or a toner having a small particle size, and in practice, there may be problems such as contamination inside the copying machines.